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Outline Background DA, NonDA NonDA Gap, DB Gap DB, DQ DO, DC, DZ 1000 Word Summary http://www.upscalegallery.com
References Background Sion, E. M., Greenstein, J. L., Landstreet, J. D., Liebert, J., Shipman, H. L., and
Wegner, G. A. 1983, ApJ, 269, 253
"A PROPOSED NEW WHITE DWARF SPECTRAL CLASSIFICATION SYSTEM"
Koester, D. and Chanmugam, G. 1990, Rep. Prog. Phys., 53, 837
"Physics of white dwarf stars"
References – DA, NonDA Barstow, M. A., Fleming, T. A., Diamond, C. J., Finley, D. S., Sansom, A. E.,
Rosen, S. R., Koester, D., Marsh, M. C., Holberg, J. B., and Kidder, K. 1993, MNRAS, 264, 16
"ROSAT studies of the composition and structure of DA white dwarf atmospheres"
Bergeron, P., Wesemael, F., Fontain, G., and Liebert, J. 1990, ApJ, 351, L21
"ON THE SURFACE COMPOSITION OF COOL, HYDROGENLINE WHITE DWARFS: DISCOVERY OF HELIUDOM IN THE ATMOSPHERES OF COOL DA STARS AND EVIDENCE FOR CONVECTIVE MIXING"
References – NonDA Gap, DB Gap Malo, A., Wesemael, F., and Bergeron, P. 1999, ApJ, 517, 901
"PHYSICAL PROCESSES IN THE ATMOSPHERES OF COOL WHITE DWARFS AND THE NATURE OF THE NONDA GAP"
Shibahashi, H. 2006, Convection in Astrophysics, Proceedings IAU Sympsium No. 239
"The DB gap of white dwarfs and semiconvection"
References – DB, DQ Althaus, L.G., Serenelli, A. M., Panei, J. A., Corsico, A. H., GarciaBerro, E., and
Scoccola, C. G., 2005, A&A, 435, 631
"The formation and evolution of hydrogendeficient postAGB white dwarfs: The emerging chemical profile and the expectations for the PG 1159DBDQ evolutionary connection"
Scoccola, C. G., Althaus, L.G., Serenelli, A. M., Rohrmann, R. D., and Corsico, A. H. 2006, A&A, 451, 147
"DQ whitedwarf stars with low C abundance: possible progenitors"
References – DO, DC, DZ Hugelmeyer, S. D., Dreizler, S., Werner, K., Krzesinski, J., Nitta, A., and Kleinman,
S. J. 2005, A&A 442, 309
"Spectral analyses of DO white dwarfs and PG 1159 stars from the Sloan Digital Sky Survey"
Harris, H. C., Dahn, C. C., Vrba, F. J., Henden, A. A., Liebert, J., Schmidt, G. D. and Reid, I. N. 1999, APJ, 524, 1000
"A VERY LOW LUMINOSITY, VERY COOL, DC WHITE DWARF" Koester, D., Wegner, G., and Kilkenny, D. 1990, ApJ, 350, 329
"MODEL ATMOSPHERE ANALYSIS OF THE DZ WHITE DWARF K78937"
Spectral Classification A second uppercase letter for secondary spectral
features Additional symbols
P – polarized magnetic star H – unpolarized magnetic star X – peculiar or unclassifiable feature(s) V – variable
Two Explanations for Observed Spectral Types
WDs can change their atmospheric composition (i.e. spectral type) as they cool “DB gap” and “nonDA gap”
Different spectral types have different progenitors Strong variation in He/H ratio in planetary nebulae
Physical Processes in the NonDegenerate Envelope
Diffusion Heavier elements sink, lighter elements float Timescale much shorter than evolutionary timescale
Should ONLY see H
Meridional circulation Caused by rapid rotation May be nonnegligible for hot WDs
Physical Processes in the NonDegenerate Envelope
Mass loss Observed for hot DAs BUT H layer is very thin and DA is most common type
Accretion of interstellar matter Too slow to affect hot DAs, but important for DZs
Physical Processes in the NonDegenerate Envelope
Radiative levitation Absorption of radiation decreases effective gravity Varies by species
Convection Probably important Species completely mixed within convection zone
DA White Dwarfs Thin H layer over much thicker He layer Exist for a large range of T
Most with T>40,000K show traces of He Those with T>50,000k also show traces of metals Most with T<40,000K show little or no He
Lower gravity stars show more nonH features => Radiative Levitation
NonDA White Dwarfs The ratio of DA/nonDA
varies strongly with T Convection
Bergeron et al. 1997, 108, 339
NonDA Gap No nonDA stars between 5100K – 6100K Cooling sequence: nonDA > DA > nonDA Seems to be an opacity effect
Not well understood
DB Gap No DBs found with 30,000K < T < 45,000K
Have DOs at T>45,000k and DBs at T<30,000K Cooling sequence: DO > DA > DB
Possibly explained by convection: At T>45,000K, have He II/III convection At T<30,000K, have He I/II convection No convection for 30,000K < T < 45,000K
DB White Dwarfs 11,000K < T < 30,000K Possible origin: “bornagain” scenario (Fujimoto,
1977, PASJ, 29, 331) Very late Heshell burning by WD remnant just before end
of H burning Thermal pulse engulfs and completely burns remaining H
envelope Leaves a hot postAGB star > DO > DB
PG 1159
DQ White Dwarfs T < 13,000K log(C/He) = 7.3 to 1.5
Due to different masses Depth of convection zone
Or different progenitors
DC White Dwarfs T < 11,000K
Old Not “white” anymore
May have H or He dominated atmosphere Spectrum may be effected by collisioninduced
absorption Collisions between H
2 and H, He atoms and molecules
induces temporary dipole (discussed last week)
DZ White Dwarfs T probably < 11,000K C, Ca, Mg, Fe, Si in a generally He atmosphere Accretion from ISM
Passage through dense interstellar cloud H/metal ratio much lower than in Sun
Somehow H is not accreted
Illarionov and Sunyaev 1975, A&A, 39, 185